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"!Gum Arabic: Past, Present and Future
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* University of Cambridge, Department of Biochemistry, Building O, Downing Site,
Cambridge CB2 1QW, United Kingdom.
24
Gum Arabic: Past, Present and Future
TIMOTHY C. BALDWIN*
AbstractAbstract
AbstractAbstract
Abstract
Plant gum exudates have been of importance in world trade for several
thousand years and still have a wide variety of practical applications
particularly in the food industry, in which they are commonly used as food
additives. These exudates are the result of the wound response to injury in
the plant species from which they are harvested. This process (gummosis)
and the molecular structure and composition of these exudates are of immense
interest and the study of which, form the basis of numerous basic and
applied research projects throughout the globe. The objective of the current
monograph is to provide a broad overview of the research data pertaining to
one of the most agronomically significant of these plant gum exudates,
namely gum arabic, and to highlight possible avenues for future research.
Key words
: Gum arabic,
Acacia senegal
, Plant gum exudate.
IntroductionIntroduction
IntroductionIntroduction
Introduction
Gum exudates from
Acacia
species commonly known as gum arabic or
gum acacia were harvested from the Gulf of Aden and exported to Egypt
as early as the seventeenth century B.C. (Whistler and BeMiller, 1993)
and are still of major commercial value today. The ancient Egyptians
refered to gum arabic in their inscriptions as ‘kami’ which was used
primarily as an adhesive for mineral pigments in paints and for the flaxen
wrappings used to embalm mummies (Whistler and BeMiller, 1993).
Through trade with Arab nations the gum gradually found its way into
European commerce and acquired the name ‘gum arabic’ from its place of
origin/port of export.
The
Acacia
species from which gum arabic is harvested, are endemic
to the Sahelian region of Africa and to date approximately 1200
Acacia
species have been identified (Ross, 1979). The commercial grades of gum
exudates obtained from these species vary considerably in quality (Islam
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et al.
, 1997). Currently, the principal source of gum arabic is the Kordofan
province of Sudan which produces over 90% of the world’s supply (Joseleau
and Ullmann, 1990) with a production figure of 40,000 tons being achieved
in 1996 (Islam
et al.
, 1997). Nigeria is the second largest producer,
followed by Chad, Mali and Senegal. Although the best quality gum is
traditionally associated with Sudan, in recent years indigenous gum from
Chad has been produced of comparable quality.
In the “gum trade”, the gum obtained from
Acacia senegal
(L.) Willdenow
has the greatest commercial value and is recognised as the best in quality.
Primarily because of this, over the past fifty or so years, most research
concerned with
Acacia
plant gum exudates has focused on the gum obtained
from this species. As a consequence of which gum arabic is currently defined
by the FAO/WHO Joint Expert Committee on Food Additives (JECFA), as
“a dried exudation obtained from the stems and branches of
A. senegal
(L.)
Willdenow or closely related species of
Acacia
(family Leguminosae)” (FAO,
1995).
Biosynthesis of Gum Biosynthesis of Gum
Biosynthesis of Gum Biosynthesis of Gum
Biosynthesis of Gum Arabic (Gummosis)Arabic (Gummosis)
Arabic (Gummosis)Arabic (Gummosis)
Arabic (Gummosis)
Gum formation (gummosis) occurs within the cambial zone of the branches
and stems of
Acacia
species and is promoted when trees of this genus are
subjected to stress conditions such as heat, drought, insect attack or
systematic wounding. Unfortunately, few rigorous investigations of this
process have been performed. The most thorough investigation to date of
gummosis in
Acacia
species was that presented by Joseleau and Ullmann
(1990). These authors noted that an extremely high molecular weight
polysaccharide, with a sugar composition very similar to that previously
observed for gum arabic was present in a zone between the inner bark and
the cambium. They speculated that this polysaccharide was the ‘precursor’
for the gum arabic exudate. These studies also indicated that gum production
in
A. senegal
requires a certain maturation of the tree. This is in agreement
with the observation in the ‘trade’ that only trees over five years of age are
able to yield an ‘economically significant’ quantity of gum.
Industrial Industrial
Industrial Industrial
Industrial ApplicationsApplications
ApplicationsApplications
Applications
The best commercial grades of gum are defined as being “clean”
i.e.
they are
highly water-soluble and give colourless or pale yellow aqueous solutions.
The gum arabic exported from Sudan is currently marketed in various
grades.
Hand Picked
Selected
is the highest grade and contains the cleanest
and largest gum nodules, with the lightest colour and therefore commands
the highest price.
Cleaned and Sifted
is the material which remains after
the Hand Picked Selected and the Siftings have been removed.
Cleaned
is
the standard grade used throughout the world, where the colour varies
from light to dark amber and the gum contains various amounts of siftings,
but has had dust removed.
Siftings
are the residue formed by sorting the
above choicer grades.
Dust
, the lowest grade is collected upon completion of
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the cleaning process, and comprises very fine particles of gum, admixed
with sand and dirt (Islam
et al.
, 1997).
These various grades of gum have a wide variety of applications. In the
food industry for example, the top grades are used to inhibit sugar
crystallization in confectionary products, in the microencapsulation of flavour
oils for use in dried food mixes such as soups, cakes etc and in the
manufacture of soft drinks (McNamee
et al.
, 1998). Gum arabic is also used
to clarify wine, as an adhesive (Anderson, 1978) and to encapsulate
pharmaceuticals (Joseleau and Ullmann, 1990). In the brewing industry
the gum is responsible for the so-called “lace-curtain effect” which develops
on the sides of beer glasses. As mentioned above, inferior grades of gum are
darker in colour and are less readily water-soluble. These lower grades of
gum are used in non-food related industries such as printing and textiles,
and in the production of explosives (Anderson, 1978).
Molecular Structure and CompositionMolecular Structure and Composition
Molecular Structure and CompositionMolecular Structure and Composition
Molecular Structure and Composition
From the extensive studies of the chemical and physicochemical properties
of gum arabic over the past fifty years, it has been shown that the gum
harvested from
A. senegal
consists primarily of polysaccharides, with
galactopyranose (Gal
p
) and arabinofuranose (Ara
f
) being the major
monosaccharides present. Gum arabic also contains a small proportion of
protein (Anderson and Stoddart, 1966). Akiyama and his colleagues
(Akiyama
et al.
, 1984) have suggested that gum arabic can be considered as
“a kind of arabinogalactan-protein” (AGP). Vandevelde and Fenyo (1985)
and Randall and colleaques (1988) have subsequently proposed that the
gum consists of at least three “fractions”. These fractions were termed an
AGP fraction (10.4% of the total gum) with molecular mass 1.45 x 106
containing 11.8% protein, an arabinogalactan (AG) fraction (88.4% of the
total) with molecular mass 2.79 x 105 containing 0.35% protein and a
glycoprotein (GP) fraction (ca. 1% of the total) with molecular mass 2.5 x
105 containing 47.3% protein. All these fractions were shown to share similar
branched structures composed of a β(1-3) linked Gal backbone with branches
of β(1-6) linked Gal containing arabinose (Ara), rhamnose (Rha), uronic acids
and their derivatives (Randall
et al.
, 1988)
Subsequently, Qi and his colleagues isolated two major fractions from
gum exudates of
A. senegal
namely a gum arabic glycoprotein (GAGP) of
high molecular weight containing ca. 90% carbohydrate (mainly Ara and
Gal), and a gum arabic polysaccharide (GAP). This GAP was shown to be a
glucorhamnoarabinogalactan with a low molecular weight and which
contained little or no protein. These workers also observed that while the
GAGP polypeptide backbone closely resembled that of extensins, the
polysaccharide moiety was similar to that of the AGPs (Qi
et al.
, 1991).
Fractionation of gum arabic by size exclusion chromatography has
further demonstrated the polydisperse nature of
A. senegal
gum components
with respect to their molecular mass and chemical composition (Osman
et
al.
, 1993a, b). This confirmed the heteropolymolecular nature of the gum
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originally proposed by Anderson and Stoddart (1966).
It is also of interest to note, in relation to the physical structure of the
molecular constituents of gum arabic, that Qi and his colleagues used electron
microscopy techniques in order to image the native GAGP molecule. This
technique revealed this molecule to consist of an extended ‘twisted hairy
rope’ like structure, similar to that previously observed for extensin
monomers (Heckman
et al.
, 1986). This data was of particular interest since
it had previously been proposed, based upon biophysical data, that AGPs
and similar molecules present in gum exudates would display a more ovoid/
spherical ‘wattle-blossom’ type of structure (Fincher
et al.
, 1983). More
recently, such ovoid structures have indeed been imaged by electron
microscopy in studies of a secreted AGP isolated from carrot suspension
culture media (Baldwin
et al.
, 1993). Therefore, since gum arabic has been
shown to contain a large number of as yet uncharacterized protein moieties
(Osman
et al.
, 1993b) it is quite feasible that it may contain both such
structures and perhaps others besides! Unfortunately, the resolution of this
intriguing question will have to await future more rigorous biochemical and
structural studies as will be discussed later.
Serotaxonomic StudiesSerotaxonomic Studies
Serotaxonomic StudiesSerotaxonomic Studies
Serotaxonomic Studies
Whilst most studies of gum arabic and related
Acacia
gum exudates (and
seed proteins) have focused purely upon the chemical, physicochemical and
structural properties of these substances, a number of chemo/serotaxonomic
investigations have also been reported.
Anderson (1978) was the first to suggest that biochemical and biophysical
data on
Acacia
gum exudates could be used to augment the classical botanical
classification of
Acacia
based solely on external morphological features such
as the ‘classic’ monograph of Bentham (1875). The first report to follow up
on Anderson’s suggestions was published by El Tinay
et al.,
(1979). In this
study, seed proteins harvested from 22 species of
Acacia
collected from
Northern, Central and Western Sudan were compared by serological methods
in an attempt to classify Sudanese
Acacia
species. From the results of
immunodiffusion and immuno-electrophoresis studies performed on these
proteins, using polyclonal antisera raised against each sample, the seed
proteins were divided into two main groups and six sub-groups. In general,
the phytochemical groupings established agreed with the botanical
classification for the main groups.
Subsequently, Brain reported a study using immunological techniques
to investigate
Acacia
phylogeny, whereby the seed proteins of 37 species of
Acacia
were tested serologically by double diffusion and immuno-
electrophoresis using rabbit antisera raised against whole seed contents of
A. karroo
,
A. ataxacantha
and
A. mearnsii
(Brain, 1987). Identity and
absorption tests showed remarkable homogeneity in the Gummiferae series.
In this study the seed proteins of
Acacias
from Africa and Australia were
analysed, and were found to have virtually identical reactions with the
antisera. In terms of the evolution and diversification of the Gummiferae
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Acacias
, it was remarkable that there was so little difference in the seed
proteins of these species despite geographical separation for millions of years
(Brian, 1987). However, the African Vulgares species studied were found to
be serologically quite distinct from the various Gummiferae samples, which
supports the view (Pedley, 1986) that the Gummiferae arose independently
from the Vulgares, and therefore should not be included in the same genus.
In relation to these and more recent serological analyses, it is of interest
to note that there is still some disagreement as to the source of gum arabic’s
immunogenicity. Some investigators have claimed that the precipitin -
forming ability of gum arabic is due to protein associated with the gum
(Akiyama
et al.
, 1984; Churms and Stephen, 1984; Connolly
et al.
, 1988).
Other workers however, have ascribed these properties to carbohydrate
residues present in the gum (Narita, 1985; Pazur and Kelly-Delacourt, 1985;
Pazur
et al.
, 1991; Miskiel and Pazur, 1991). Bearing in mind the relatively
low levels of protein present in gum arabic, and given the proven antigenicity
of arabinogalactans and AGPs, both of which are abundant in samples of
gum arabic (Anderson
et al.
, 1984; Knox
et al.
, 1992), it would therefore
seem most probable that the carbohydrate moeities present in gum arabic
comprise the dominant immunogens present. .
. .
. Indeed, Miskiel (1990) has
determined two immunodeterminant groups present in gum arabic to be α-
L-arabinofuranosyl-(1→4)-D-galactose and β-D-glucuronopyranosyl-(1→6)-
D-galactose.
The first detailed description of the use of anti-gum arabic antibodies in
an immunoassay was described by Pazur (1986). Since which time several
similar reports have also been published (Pazur
et al.
, 1991; Miskiel and
Pazur, 1991; Williams
et al.
, 1992; Menzies
et al
, 1992; Osman
et al.
, 1993a).
Recently, a sensitive and specific ELISA for
A. senegal
gum has been reported
which could differentiate this gum from other commonly used food
hydrocolloids including other plant exudates such as gums ghatti, tragacanth
and karaya (Williams
et al.
, 1992; Menzies
et al.
, 1992). Further studies of
Acacia
gums using this ELISA in combination with a range of chemical and
physicochemical techniques indicated that the interaction with the anti-
gum arabic antisera could be correlated with differences in the molecular
composition of the gums. These studies suggested the usefulness of such an
immunoassay in chemotaxonomic studies of
Acacia
(Osman
et al.
, 1993a).
The only major drawback of this technique was that only a finite quantity
of the polyclonal antisera was available. However, in the same year Osman
and his colleagues also reported the use of a panel of anti-arabinogalactan/
arabinogalactan-protein (AG/AGP) monoclonal antibodies in conjunction
with a range of chemical/physicochemical analyses to study the molecular
composition of gum arabic (Osman
et al.
, 1993b). This and later studies
(Osman
et al.
, 1995; Menzies
et al.
, 1996; Baldwin
et al.
, 1999) clearly
demonstrated the utility of anti-plant cell wall monoclonal antibodies in
analyses of the macromolecular composition of gum arabic and fractions
derived from it, and also the ability of such antibodies to distinquish between
gum exudates harvested from a wide variety of
Acacia
species. The ability
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to distinquish quickly and cheaply between gums harvested from various
species of
Acacia
via such immunological tests could be of immense benefit
to quality control agencies in the food industry since consignments of gum
arabic are in some cases ‘doctored’ with potentially dangerous ‘non-food grade’
gums.
Future ProspectsFuture Prospects
Future ProspectsFuture Prospects
Future Prospects
As mentioned previously, the primary challenge that faces future basic
scientific investigations in this field, concern the biosynthesis and
macromolecular composition of the gum. Unfortunately, the ‘traditional’
sources of natural biopolymers such as gum arabic are subject to the
vagaries of the weather, plant disease and local politics, all of which
inevitably lead to variations in quality and supply problems. These problems
would be greatly reduced, and the molecular synthesis and structure of the
gum would become more ammenable to study, if it could be produced in
large quantities using biotechnological methods such as plant cell culture.
Using this technique the plant cells would be grown in large fermenters
and the gum would be secreted into the liquid culture media, from which it
would subsequently be extracted. Indeed such projects are already underway
in a number of scientific institutions throughout the globe such as “The
Cooperative Research Centre for Industrial Plant Biopolymers” based at
the University of Melbourne, Australia.
Hopefully, these technologies once established will also be transferred
to ‘traditional’ areas of gum production in developing countries, so that these
countries do not suffer from a catastrophic loss of revenue once this
technology becomes commonplace.
The eventual industrial scale production of gum arabic by such methods
would thereby facilitate more rigorous studies of the molecular composition
and biosynthesis of the gum. Once the molecular and biochemical basis of
gummosis is better understood it may then be possible to genetically engineer
the plant cells in such a way as to produce biopolymers with enhanced
properties specific to their final end use. As yet this is still in the realms of
“blue skies” research, but it is fascinating to speculate how this and other
future technologies will further our understanding and knowledge of this
unique and remarkably useful natural product.
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